Audrey Le Pioufle15 October 2018

Did you know that part of Australia was once a part of Canada? Amazing, isn’t it?

This discovery was revealed by a team of geologists in a 2018 article. Geologists are often referred to as the historians of the Earth. For many years they have developed tools and techniques to unravel the secrets of the Earth’s history.

In this article, we will talk about supercontinents, and how geologists use a technique called radiometric dating to study them. Then we’ll talk about how geologists used this technique to discover a piece of Canada down under.

Supercontinents

A supercontinent is an assembly of all (or most) landmasses into one very large continent. Supercontinents happen when, because of plate tectonics, pieces of the lithosphere (Earth’s outer shell) move over the Earth’s mantle into one huge landmass.

Throughout geological time, there have been several supercontinents on Earth. For example, Pangea, the most recent supercontinent, existed around 300 million years ago. Another supercontinent was Nuna (Columbia), which formed about 1600 million years ago, during the Precambrian era. In order to discover past supercontinents, scientists must work with a number of tools that tell us Earth’s history.

Did you know? The geological timescale is the history of the Earth divided in several intervals known as eons, eras, periods, epochs and stages. Each interval corresponds to major geologic events in the history of our planet [5] [6]. For example, the extinction of the dinosaurs 65 millions years ago represents the Mesozoic age (age of the reptiles) to Cenozoic age (age of the mammals).”

Radiometric dating

One example of these tools to study Earth’s history is geochronology, the study of the age of rocks. With geochronology, geologists can calculate the age of rocks by using radioactive elements. Examples include uranium, thorium, potassium, and carbon.

An element that exists with different numbers of neutrons is said to have isotopes. Isotopes can be stable or unstable. A stable isotope does not decay into other elements. An example of a stable isotope is 13C (carbon 13), which has 6 protons and 7 neutrons.

Meanwhile, unstable isotopes naturally decay in the environment and change atomic structure, whether it be their proton number, their neutron number or both. The original isotope, called parent isotope, decays into a daughter isotope. For instance, a parent isotope could be 14C, which decays into 14N. In other words, carbon 14, which has 6 protons and 8 neutrons, decays into nitrogen 14, which has 7 protons and 7 neutrons.

All radioactive elements decay at a constant rate. This rate is called half-life. The half-life of a radioactive element is the time it takes for half the amount of the parent isotope to decay into the daughter isotopes.

This constant rate of decay acts as a clock. Geologists can use these clocks to calculate the age of a rock. They measure the amount of parent isotopes left and of daughter isotopes that have been generated [4]. For instance, one may use the ratio 206Pb/238U (lead 206 to uranium 238) in relation to 207Pb/235U (lead 207 to uranium 235) to calculate an age. Geologists use the amount of parent isotopes in comparison to the amount of daughter isotopes to determine the age of the rock. You can learn more at this link.

Did you know? A mass spectrometer is an instrument that ionizes compounds and separates the elements within them according to their mass to charge ratio. It allows scientists to analyze a wide variety of samples. [3]

How did scientists use this technique in Australia?

Scientists used the U/Pb dating method to date the age of sedimentary rocks in Georgetown Inlier, Northeast Australia [1]. They looked at the abundance of U and Pb ratio in the rocks. Using this technique, they discovered that these rocks have the same age as the Wernecke Supergroup, sedimentary rocks found in the Yukon and the Athabasca Basin in Canada! These rocks were formed from the Laurentia supercontinent that existed 1.7-1.6 billion years ago.

The Georgetown Inlier rocks ages cannot be linked to any other rocks in Australia. This suggests that the Australian segment formed on the Laurentia continent and drifted apart during a tectonic event around 1680 million years ago. The authors suggest that the continent collided with Australia around 1600 million years ago, when the Nuna supercontinent formed. When Pangea broke apart, some 300 million years ago, the Georgetown Inlier remained with Australia.

The work of geologists

When a discovery like this is made, still have to use other dating methods to cross-check the findings. As you can see, reconstructing the past, and the history of supercontinents, is a difficult task, but the historians of the Earth are working hard to unravel its history. Geochronology is an accurate and useful tool to accomplish such a feat.

This article published in Geology reminds me of my bachelor degree in Earth Sciences. Back then, I worked in the geochronology laboratory of my department. At 19 years old, I worked with zircons from the extraction process to the dating steps on the mass spectrometer.

This article made me nostalgic. And it also reminded me to remind you that this field of study is easily reachable!

Audrey Le Pioufle

My name is Audrey and I am a geoscientist-by-training. Since my childhood, I’ve always been attracted to nature and rocks. To realize my longtime dream of becoming an Earth Scientist, I first completed a B.Sc. in Geology in 2009 at the Université du Québec à Montréal in Québec. In 2010, I pursued a M.Sc. in experimental petrology at the University of Victoria, in British Columbia, in a pursuit of accomplishment; however, when I graduated my curiosity was yet to be completely fulfilled. At that time, I realized that learning was in fact a primary component of my personal development. I am now pursuing a PhD degree in Earth sciences at the Institut National de la Recherche Scientifique (INRS) in Québec City. I hope, through my life experiences, to share my passion for sciences.